In recent years, new energy-production techniques have attracted attention from the standpoint of environmental issues, and among these techniques a fuel cell has attracted particular interest. The fuel cell converts chemical energy to electric energy through electrochemical reaction of hydrogen and oxygen, attaining high energy utilization efficiency. Therefore, extensive studies have been carried out on realization of fuel cells for civil use, industrial use, automobile use, etc.
Fuel cells are categorized in accordance with the type of employed electrolyte, and a phosphate type, a fused carbonate salt type, a solid oxide type, and a solid polymer type have been known. With regard to hydrogen sources, studies have been conducted on methanol; liquefied natural gas predominantly containing methane; town gas predominantly containing natural gas; a synthetic liquid fuel produced from natural gas serving as a feedstock; and petroleum-derived hydrocarbons such as LPG, naphtha, and kerosene.
When hydrogen is produced from these gas or liquid hydrocarbons, the hydrocarbons are generally partial-oxidation-reformed, autothermal-reformed, or steam-reformed, in the presence of a reforming catalyst.
When a hydrocarbon fuel such as LPG, town gas, or kerosene is reformed so as to produce hydrogen serving as a fuel, the sulfur content of the hydrocarbon fuel must be reduced to 0.1 ppm or lower in order to prevent poisoning of the reforming catalyst. When a hydrocarbon such as propylene or butene is employed as a feedstock for petrochemical products, the sulfur content of the hydrocarbon must be reduced to 0.1 ppm or lower in order to prevent poisoning of the reforming catalyst.
The aforementioned LPG generally contains sulfur compounds such as methylmercaptan and carbonyl sulfide (COS), and an odorant such as dimethylsulfide (DMS), t-butylmercaptan (TBM), or methyl ethyl sulfide is intentionally added thereto. Recently, research efforts have been devoted to utilization of oxygen-containing hydrocarbon compounds, such as dimethyl ether, as a fuel. Although no sulfur compound is included in the oxygen-containing hydrocarbon compounds, studies have been conducted on intentional addition of the aforementioned odorant to the hydrocarbon compounds, because the odorant would effectively warn gas leakage.
There have been known a variety of adsorbents which adsorb sulfur compounds contained in a hydrocarbon fuel such as LPG or town gas so as to remove the compounds from the fuel. Although some of these known adsorbents exhibit excellent desulfurization performance at about 150 to about 300° C., currently attained desulfurization performance at ambient temperature is not satisfactory.
There have been disclosed desulfurizing agents; for example, desulfurizing agents containing hydrophobic zeolite and a metallic element such as Ag, Cu, Zn, Fe, Co, or Ni carried thereon through ion-exchange (see, for example, Japanese Patent Application Laid-Open (kokai) No. 2001-286753) and desulfurizing agents containing Y-, β-, or X-type zeolite and Ag or Cu carried thereon (see, for example, Japanese Patent Application Laid-Open (kokai) No. 2001-305123). These desulfurizing agents effectively adsorb, at room temperature, mercaptans and sulfides contained in a fuel so as to remove the sulfur compounds from the fuel, but adsorb virtually no carbonyl sulfide.
Copper-zinc desulfurizing agents are also disclosed (see, for example, Japanese Patent Application Laid-Open (kokai) No. 2-302496). Although the desulfurizing agents adsorb a variety of sulfur compounds at 150° C. or higher so as to remove the compounds, sulfur compound adsorption performance at 100° C. or lower is unsatisfactory. Also disclosed is a desulfurizing agent containing a porous carrier (e.g., alumina) and copper carried thereon (see, for example, Japanese Patent Application Laid-Open (kokai) No. 2001-123188). The desulfurizing agent can also be employed at 100° C. or lower, but its adsorption performance is not sufficient.